Re: PROBLEM: WARN_ON triggers in v4l_querycap with BlackMagic atem console connected via USB

[Hans Verkuil]

Hi Ulrich,

What is the output of 'v4l2-ctl -D -d/dev/videoX' of the video device that causes
the WARN_ON to appear? I can't tell what driver is used.

The WARN_ON just checks if the capabilities field is a superset of the device_caps
field. If not, then that's a driver bug. If this is an out-of-tree driver, then
it has to be addressed there. Note that that WARN_ON has been there since mid-2019.

Regards,

	Hans

On 06/10/2024 12:50, Ulrich Drepper wrote:
> I cannot really say when this problem started but I know that perhaps
> six months back (or a bit more) I used the console successfully as a
> video source device (webcam).  This is a ATEM Mini Pro which has a
> large number of USB interfaces.  What trips up is videodev.  Various
> userlevel programs (uvcdynctrl, chromium) notice the device being
> added and use ioctl(), triggering the problem.
> 
> This is with the current Fedora x86_64 kernel (6.10.12) but a) it
> happens for a while now and b) as far as I can see this is just the
> upstream code, no local changes.
> 
> The call trace is:
> 
> Call Trace:
>  <TASK>
>  ? v4l_querycap+0x119/0x140 [videodev]
>  ? __warn.cold+0x8e/0xe8
>  ? v4l_querycap+0x119/0x140 [videodev]
>  ? report_bug+0xff/0x140
>  ? handle_bug+0x3c/0x80
>  ? exc_invalid_op+0x17/0x70
>  ? asm_exc_invalid_op+0x1a/0x20
>  ? v4l_querycap+0x119/0x140 [videodev]
>  __video_do_ioctl+0x518/0x630 [videodev]
>  video_usercopy+0x1f1/0x7a0 [videodev]
> 
> I traced the offending code to an 'ud2' instruction (makes sense,
> undefined) which is reached from this code in
> drivers/media/v4l2-core/v4l2-ioctl.c (v4l_querycap):
> 
> WARN_ON((cap->capabilities &
>       (vfd->device_caps | V4L2_CAP_DEVICE_CAPS)) !=
>       (vfd->device_caps | V4L2_CAP_DEVICE_CAPS));
> 
> The asm code is:
> 
>     747e:       8b 73 54                mov    0x54(%rbx),%esi
>     7481:       81 ca 00 00 00 80       or     $0x80000000,%edx
>     7487:       48 b9 00 00 20 00 00    movabs $0x20000000200000,%rcx
>     748e:       00 20 00
>     7491:       48 0b 4b 54             or     0x54(%rbx),%rcx
>     7495:       21 d6                   and    %edx,%esi
>     7497:       39 f2                   cmp    %esi,%edx
>     7499:       75 6e                   jne    7509 <v4l_querycap+0x119>
> 
> where 7509 is the address of the ud2 instruction.
> 
> The register dump shows
> 
> RDX: 0000000085008003 RSI: 0000000085008002
> 
> which, if I'm right, means that cap->capabilities has the extra bit
> 
> #define V4L2_CAP_VIDEO_CAPTURE          0x00000001  /* Is a video
> capture device */
> 
> set.
> 
> I don't know what 'ops' points to in the code.  It seems to me that
> the bit is set in the
> 
> ops->vidioc_querycap(file, fh, cap);
> 
> call just preceding the test.
> 
> Just going by the name of the capability, it seems that the bit should
> be set as this device is used as a camera as well so maybe the
> information which comes from the call the
> 
> struct video_device *vfd = video_devdata(file);
> 
> is incomplete?  Or the test WARN_ON condition is actually incorrect?
> 
> 
> Any idea?  I can try to run more tests if someone can tell me how to proceed.
> 
> Thanks.